This invention relates to electrode systems of the type that can be surgically implanted in the human body and attached to nerve bundles to provide electrical stimulation for those nerves and a means for recording electrical activity that occurs naturally in nerves.
The use of electrical currents to effect, control, and detect the activation of individual nerves constitutes an important tool for research on the function of the nervous system and for the treatment of neurological disorders stemming from paralysis, spasm, and other dysfunctional states of the organs innervated by such nerves. The clinical applications of electrically applied nerve stimulation include many of those that are the subject of current research in the field of neural prosthetics, including the functional electrical stimulation and blocking of peripheral nerves controlling the muscles of the arms and legs and the pelvic viscera such as the bladder and colon and their sphincteric muscles. Other applications of such an electrode include the recording of naturally occurring activity in intact nerves for purposes of obtaining motor control and sensory feedback signals.
In such applications, the efficiency and selectivity whereby a particular nerve bundle is recorded or stimulated, to the exclusion of surrounding nerves and other electrically active structures, depends on two separate electromechanical relationships. First, the electrode contacts which convey the electrical signals between the nerve and associated electronic circuitry must be positioned close to and surrounding the nerve bundles at two longitudinally spaced-apart positions. This causes the electrodes to detect or generate current flow along the axis of the nerve fibers comprising the bundle, in conjunction with natural propagation patterns which constitute the conducted action potentials of the individual nerve fibers. Second, the electrical fields in the vicinity of the nerve and the electrode contacts need to be confined to the longitudinal cylinder of the nerve itself, to prevent their dissipation by the volume conducting properties of the surrounding tissues and fluids.
Devices of the prior art have attempted to achieve both of these goals with cylindrical enclosures that completely or nearly completely surround the nerve and provide support for the electrode contacts and their attached electrical leads. This has caused such devices to be unnecessarily bulky, difficult to implant reliably, and prone to cause damage by compressing or kinking either the nerve or its associated blood supply or both of them. Furthermore, such devices must be carefully constructed to the dimensions of each anatomical configuration, and cannot be adapted or adjusted during surgery to cope with unexpected dimensions or configurations.
It is therefore the primary object of the present invention to provide a superior surgically implantable electrode system for providing electrical connection to a nerve bundle.
It is another object of the present invention to provide an effective method of attaching the electrode system to a nerve bundle during surgery without special instruments or techniques.
It is yet another object of the present invention to provide an implantable electrode system which, due to its unique construction, is easily adjusted by the surgeon at the time of implantation to accommodate nerves of any dimension or anatomical configuration and which provides precise and constant spatial relationships between the electrode contacts and the nerve bundle.
It is a further object of the present invention to provide an implantable electrode system which insures electrical isolation so that even closely adjacent, multiple sites of electrical recording or stimulation interact minimally.
It is a still further object of the present invention to provide an implantable electrode system whose physical properties and overall construction impose minimal trauma on the nerve and the device at the time of implantation, during the development of any scar tissue, and during movement of the device and the body parts in normal use.
The aforementioned objects of the present invention are accomplished by providing an electrode structure having a helically shaped insulative carrier or substrate with two or more electrical contacts imbedded on the inner-facing surface thereof. Electrical lead-ins are connected to the contacts and travel within the carrier to the end thereof where they exit as a bundle of wires running substantially parallel to the nerve bundle. This helical electrode structure can be slipped over the nerve by rotating it like a corkscrew. The pitch of this helix is more shallow and the turns more widely spaced than helical electrodes designed according to prior art, making the invention easier to apply and less damaging to delicate nerve bundles. For such a design to satisfy the object of stable, isolated electrical connection with the nerve bundle, separate means are provided as follows for confinement of electrical current around the nerve and electrode contacts and for strain relief of the electrical leads. The confinement of the electrical currents generated by or supplied to the nerve is accomplished by wrapping the nerve and the wound electrode structure in a thin, double layered, sheet or membrane of electrically nonconductive, flexible material such as silicone rubber. The layers can be closed by the use of conventional surgical sutures, staples, adhesive or the like. Strain relief of the electrode structure that would otherwise tend to drag on the nerve is accomplished by providing a slidable tab made of a stiff elastic material that can be clamped on the electrical leads. The tab frictionally engages the lead-in conductors and is attached to bone, fascia or other suitable connective tissue by screws, sutures, staples or other surgical fasteners. A plurality of electrode structures can be maintained in spaced apart relationship on a single, common base of insulative sheet or membrane material, having a plurality of separate covers of the same material, each adapted to enclose a separate electrode structure.